Underlying mechanisms of resistance drift in amorphous phase-change materials
Aachen (2020) [Dissertation / PhD Thesis]
Page(s): viii, 156, A25, XXXII Seiten : Illustrationen, Diagramme
Phase-change materials (PCM) are poised to play a key role in next-generation storage systems, as they are currently the most promising candidate for bridging the gap in access time between memory and storage in traditional memory hierarchy. These materials can be switched reversibly between an electrically highly-resistive amorphous (RESET) and a highly-conductive crystalline (SET) state, storing the two states of a bit. With respect to their integration in non-volatile memories, the retention of a once programmed RESET state is impaired by a steady increase in electrical resistance over time. While this so-called resistance drift is commonly related to structural relaxation of the amorphous phase, its exact underlying mechanisms remain unclear until today, which has provoked the engineering of elaborate workaround solutions. The present work aims at contributing to research on the underlying mechanisms of resistance drift, in order to improve competitiveness of electronic phase-change memory by means of profound material research. More precisely, the effect of structural relaxation on electrical transport in the semiconducting amorphous phase of PCM is investigated, focussing on three coherent sub-studies. First, the temporal evolution of infra-red reflectance spectra is measured on amorphous thin films of the three PCM AgIn-doped Sb2Te, GeTe and the most popular Ge2Sb2Te5. Complemented by data sets recorded during temperature sweeps, comprehensive information on the bandgap are extracted and interpreted within the band transport model for electrical transport. The low-drift characteristic of AgIn-doped Sb2Te motivates the second sub-study, which focusses on field-dependent transport modelling of IV curves recorded during relaxation on a nano-scale PCM memory device. Since field-dependent conductivity is largely affected by the interaction of charge carriers with localised states, it is examined to what extent transport modelling including corresponding trap- and release events of charge carriers can be utilised to learn about the evolution of the electronic density of states (DOS) upon relaxation. In the third sub-study, a current lack of knowledge of the DOS and capture characteristics of localised states in amorphous AgIn-doped Sb2Te is addressed by employing advanced modulated photocurrent (MPC) spectroscopy. Eventually, the present work reveals bandgap widening upon relaxation as fingerprint in amorphous PCM. However, a quantitative comparison with experimental data for the apparent activation energy of electrical conduction indicates that the temporal evolution of bandgap and activation energy is decoupled in AgIn-doped Sb2Te. This demonstrates the possibility to identify new PCM with reduced resistance drift and might be explained by a beneficial disappearance of localised states attributed to a structural defect. Such a defect might be positioned near the conduction band, since experimental results show no pronounced DOS peak below the Fermi level. In view of the absence of a such a defect serving as main channel for carrier emission and clearly non-constant capture characteristics observed in the valence bandtails of Ge2Sb2Te5 and AgIn-doped Sb2Te, necessary modifications to the current Poole-Frenkel-based transport modelling are proposed. Eventually, not only the disappearance of localised states in the bandgap upon relaxation, but also a change in capture characteristics of localised states is suggested as underlying mechanism of resistance drift.
Rütten, Martin Werner